FDA regulation of the safety of MR devices: past, present, and future.

The history of the Food and Drug Administration's (FDA) regulation of safety issues for MR devices is reviewed in this article. The guidance documents and policies that FDA has used from 1982 to the present to ensure safety are summarized. The present status of FDA regulatory policies and apparent plans for the future are included.

Effectiveness of latex condoms as a barrier to human immunodeficiency virus-sized particles under conditions of simulated use.

Condoms were tested in an in vitro system simulating key physical conditions that can influence viral particle leakage through condoms during actual coitus. The system quantitatively addresses pressure, pH, temperature, surfactant properties, and anatomical geometry. A suspension of fluorescence-labeled, 110-nm polystyrene microspheres models free human immunodeficiency virus (HIV) in semen, and condom leakage is detected spectrofluorometrically. Leakage of HIV-sized particles through latex condoms was detectable (P less than 0.03) for as many as 29 of the 89 condoms tested. Worst-case condom barrier effectiveness (fluid transfer prevention), however, is shown to be at least 10(4) times better than not using a condom at all, suggesting that condom use substantially reduces but does not eliminate the risk of HIV transmission.

PIP: Physical science researchers tested the ability of 89 undamaged latex condoms manufactured in the US to prevent passage of HIV=size particles under simulated physiologic conditions at their Food and Drug Administration laboratory in Rockville, Maryland. The design of the test system considered particle size, pH, surface tension, and time. A suspension of polystyrene 110 nm microspheres labeled with fluorescent dye served as the HIV-sized particle model in semen. They challenged each condom with this suspension for 30 minutes. The test did not include motion since stretching over the penis accounts for most pore stretching. Leakage of fluorescent dye occurred in 29 condoms (p .03). 21 condoms leaked at minimum leak rates 1 nl/s, 7 at 1-6 nl/s, and 1 at around 10 nl/s. Assuming the leakage occurred through the only pore in each condom, the pore diameters ranged from 2 to 7 mcm. Also assuming an even more conservative criterion, the qualitative results were the same: 11 condoms with leak rates were nl/s vs. 6 condoms with leak rates 1-9 nl/s (p .002). The widely used 300 ml water test did not indicate any pores in any of the condoms. In the extreme and highly unlikely scenario of all the fluid being pumped out of the condom, the transfer rate would be about 0.1 mcl after 10 minutes of thrusting after ejaculation filled the condom with semen (i.e., 0.01% of a typical 3 ml ejaculate). Thus proper use of latex condoms would result in exposure reduction from HIV of at least 4 orders of magnitude. These findings demonstrated that use of latex condoms can significantly reduce the risk of HIV transmission, but it does not eliminate that risk.

Test method for evaluating the permeability of intact prophylactics to viral-size microspheres under simulated physiologic conditions.

The alarming number of AIDS cases has increased the attention given to barrier devices such as condoms. The authors describe a new test method that evaluates the permeability of the intact condom when subjected to simulated physiologic conditions. Fluorescent-labelled polystyrene microspheres (110 nm diameter) are used to model cell-free virus. Physical and chemical conditions that are present during coitus, such as pressure, pH, and temperature, are considered in the design of the method. The testing chamber is designed to be continuously monitored for changes in fluorescence due to leakage across the condom surface. The sensitivity of the system is 1 x 10(-5) of the original concentration of microsphere solution (3.4 x 10(11) particles/mL), which corresponds to leak rates as small as .001 microL/sec. The test provides an in vitro test of barrier material permeability relevant to actual use.

Action potential-induced fluorescence changes resolved with an optical fiber carrying excitation light.

With the use of a single, implantable, optical fiber, to excite fluorescence and detect changes from voltage-sensitive dyes, transmembrane potential changes were measured without the need for a clear line-of-sight path between the excitation light, the tissue, and the detector. In a previous study, we were required to use signal averaging and could detect only cardiac action potentials from frog. In the present study we improved this system so that unaveraged cardiac action potentials were resolved with high fidelity, and action potentials from single nerve axons were detected. Endeavors to optimize the signal-to-noise ratio resulted in the selection of a larger core fiber with a rounded tip, styryl dyes, and filters based upon fluorescence spectra of the dyes when bound to membrane (rather than in solution). The frog gave signals nearly comparable in magnitude and signal-to-noise ratio to those seen with systems that use a fluorescence microscope. Action potential-induced signals could be detected in single lobster axons with the intracellular injection of a dye. The improvement in the signal-to-noise ratio allowed the use of a reduced-intensity excitation illumination which produced less bleaching of the dye.

A model of the temperature rise in the head due to magnetic resonance imaging procedures.

A simple model of power deposition in the head is presented in which the temperature increase in the head in general and, in particular, in a small spherical region of unperfused tissue contained within the head is calculated. The result of the calculation provides a limit on the SAR needed to ensure that a 1 degree C temperature rise is not exceeded for unperfused spheres of different sizes, imaging sessions of different lengths of time, and different blood flow rates to the head. The model assumes that the rest of the body during a head imaging session can act as a perfect heat sink at 37 degrees C. The model suggests that current practices in MRI imaging will not cause a temperature rise in the center of small unperfused regions such as the eye of more than 1 degree C.

Specific absorption rate (SAR) in models of the human head exposed to hand-held UHF portable radios.

Specific absorption rate (SAR) was measured in models of the human head exposed to hand-held portable radios ("transceivers") transmitting at frequencies in the 800-MHz band. An isotropic implantable electric-field probe was used to measure internal fields induced in the head models, and SARs were determined by calculation. As well as determining representative values and distributions for SARs under various conditions, it was shown that antenna type and orientation with respect to the head are important factors affecting energy absorption.

Comparative study of shortwave heating patterns in phantoms with polyethylene and silk partitions.

Specific absorption rate (SAR) and effective depths of heating patterns induced by a shortwave, pancake diathermy applicator in fat-muscle phantom are measured. Midplane partitions of polyethylene and silk screen with and without contact chemicals are used. Thermographically obtained SAR data show nearly the same value for silk-screen partitions with and without contact chemicals and slightly lower values with polyethylene partitions, provided that the partition midplanes are tightly pressed against each other. Thermometry data indicate that for low-power exposures the major error in thermographic measurements obtained after termination of heating is due to thermal diffusion and not evaporative cooling in the opened midplane of the phantom.

Magnetic fields associated with a nuclear magnetic resonance medical imaging system.

Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures.

Comparison of RF-induced calcium efflux from chick brain tissue at different frequencies: do the scaled power density windows align?

The recent suggestion that the pattern of positive and negative results of RF-induced calcium efflux from chick brain tissue, when appropriately scaled, matches at three frequencies is examined. Close scrutiny of this recently reported analysis by Joines and Blackman suggests that the uncertainties in the calculated scaling quantities are too large to permit meaningful conclusions.

Dielectric properties of animal tissues in vivo at frequencies 10 MHz--1 GHz.

An open-ended coaxial line sensor in conjunction with an automatic network analyzer was used to measure in vivo the permittivity of several feline tissues (skeletal and smooth muscle, liver, kidney, spleen, and brain--gray and white matter) at frequencies between 10 MHz and 1 GHz. The estimated uncertainties of measurement were between 1.5% and 5%. The data are in general agreement with previously obtained data in vitro and in vivo. Significant differences in the properties of different types of the same tissue (eg, skeletal and smooth muscle) were observed. Many tissues were found to be non-homogeneous in its permittivity.